1. Field of the Invention
The present general inventive concept relates to a switching mode power supply and a method of operating a power supply in a power save mode. More particularly, the present general inventive concept relates to a switching mode power supply that outputs at least two voltages and operates in a power save mode using a voltage difference when a minimum load is applied, and a method of operating in a power save mode.
2. Description of the Related Art
Since a conventional switching mode power supply (SMPS) operates a switching element in a switching mode, it consumes less power. Also, since the SMPS uses a high frequency power transformer, the SMPS is compact-sized and light-weight. It is possible for the SMPS to output different levels of direct current (DC) voltages at the same time. For example, if a printer employs the SMPS, it is possible for the printer to be supplied with a 3.3V or 5V (DC) voltage from a main power supply and a 24V (DC) voltage supplied from a high voltage power supply or a printer engine at the same time.
The SMPS enters a power save mode to minimize power consumption when the high voltage power supply or the printer engine that uses a high voltage is idle.
The conventional SMPS has at least two pulse width modulation (PWM) controllers and at least two transformers to output at least two voltages. The conventional SMPS receives a PWM on/off signal to control the output of the high voltage to turn off the second PWM controller such that the high voltage is not output in the power save mode.
More specifically, if a SMPS is designed to output two voltages for example, the SMPS includes two PWM controllers and two transformers.
It is assumed that a first PWM controller controls a low voltage output and a second PWM controller controls a high voltage output.
The first PWM controller outputs a PWM signal, and a first transistor switches on/off according to the PWM signal and thereby regulates a current flowing through a primary coil of a first transformer, and thus controls a voltage induced at a secondary coil of the first transformer. The voltage induced at the secondary coil at the first transformer is rectified, smoothed, and then output to a first output terminal as the low voltage. The second PWM controller and a second transistor generate a voltage in the same manner as described above and output the voltage to a second output terminal as the high voltage.
The conventional SMPS requires an external controller to output the PWM on/off signal to control the high voltage output from the second PWM controller to turn on/off (i.e. from on to off) when entering a power save mode. When receiving the PWM on/off signal from the external controller, the second PWM controller is shut down according to the received PWM on/off signal. That is, the second PWM controller stops outputting the high voltage when the PWM on/off signal is received.
That is, the SMPS has to receive the control signal in order to operate in the power save mode. Accordingly, the SMPS also requires the external controller to output the control signal.